A variable gain amplifier (VGA) is an analog circuit that can be used in a front-end receiver in a communication device to amplify or attenuate received signals. For example, when strength of a radio frequency (RF) signal received by the receiver is relatively low, the variable gain amplifier may amplify the RF signal prior to the RF signal being down-converted to a lower frequency. Also, for example, when the strength of the received RF signal is relatively high, the variable gain amplifier may provide increased linearity to the RF signal by decreasing the gain.
FIG. 1 illustrates a conventional variable gain amplifier 100. The variable gain amplifier 100 includes a pair of differentially connected metal-oxide-semiconductor (MOS) transistors M1 102 and M2 104. The MOS transistor M1 102 includes a gate terminal 106, a drain terminal 108, and a source terminal 110. The MOS transistor M2 104 includes a gate terminal 112, a drain terminal 114, and a source terminal 116. The gate terminal 106 of the MOS transistor M1 102 is coupled to a first input terminal 118 of the variable gain amplifier 100. The gate terminal 112 of the MOS transistor M2 104 is coupled to a second input terminal 120 of the variable gain amplifier 100. The drain terminal 108 of the MOS transistor M1 102 is coupled to a first resistor 122 with an adjustable resistance value and a first output terminal 124 of the variable gain amplifier 100. The drain terminal 114 of the MOS transistor M2 104 is coupled to a second resistor 126 with an adjustable resistance value and a second output terminal 128 of the variable gain amplifier 100. The source terminal 110 of the MOS transistor M1 102 and the source terminal 116 of the MOS transistor M2 104 are connected together to be driven from a constant current source Is 130. By adjusting the resistance values of the first and second resistors 122 and 126, the amplifier 100 may provide a variable gain to a first voltage Vin applied at the input terminals 118 and 120, and output a second voltage Vout at the output terminals 124 and 128.
Typically, distortion is a consideration in the design of the variable gain amplifier. It is desired that the variable gain amplifier has a wide linear input range. For example, when the receiver receives a signal on a desired frequency channel, it may also receive an interference signal on an adjacent frequency channel, which may have a detrimental effect on the signal received on the desired frequency channel. If the variable gain amplifier has a wide linear input range, the variable gain amplifier may provide increased linearity to the received signal by decreasing the gain. As a result, the variable gain amplifier with a wide linear input range may provide an increased signal-to-noise/interference ratio to its output signal.